Originally Posted by

**lil_cookie** This is my practice exam question and I have no answer for it nor do I know how to go about this....any help is greatly appreciated.

An open-topped cylindrical water reservoir has a volume of $\displaystyle 320{\pi}m^3$.

The material used for the bottom of the reservoir costs $5/$\displaystyle m^2$ and the material used for the vertical portion costs $8/$\displaystyle m^2$.

Determine the radius and the height of the cylinder so that the cost is a minimum.

They also provide me with these equations:

Volume = $\displaystyle {\pi}r^2h$

surface are = $\displaystyle {\pi}r^2 + 2 {\pi}rh$